Running queries on system tables

Learn how to configure system table queries in Impala to use only the necessary coordinator resources.

Queries against Impala system tables, such as sys.impala_query_live, experienced delays due to admission control constraints. These queries, which only require only coordinator resources, were blocked by other queries competing for executor resources.

To address this, Impala introduces an "only coordinators" request pool. This allows system table queries to run without waiting for executor resources, even when those resources are fully used. In Cloudera Data Warehouse, queries submitted to an only coordinators request pool continue to run even if no executors are running.

Coordinator-only request pools

You can configure coordinator-only request pools in Impala by setting the <onlyCoordinators> option to true in the fair-scheduler.xml file. When enabled, the request pool runs queries only on coordinators. No executors are required, and all fragment instances are executed exclusively on the coordinators.

To prevent resource issues, Cloudera recommends limiting access to coordinator-only request pools to a small group of users who understand the risks of running complex queries in this configuration.